1. Field of the Invention
The present invention relates to an operating circuit applied to a backlight, and more particularly, to an operating circuit applied to a light-emitting diode (LED) backlight.
2. Description of the Prior Art
Please refer to FIG. 1, which illustrates a prior art backlight module control system 100. As shown in FIG. 1, the backlight module control system 100 includes a plurality of LED strings 110_1-110_4, a plurality of current control circuits 120_1-120_4, a minimum voltage selector 130 and a voltage booster 140, where each of the LED strings 110_1-110_4 includes a plurality of LEDs, and each of the current control circuits 120_1-120_4 includes an amplifier (122_1-122_4), a transistor (M1-M4) and a resistor R.
Because each of the LEDs has a semiconductor process variation, the cross voltages of the LED strings 110_1-110_4 are different while the LED strings 110_1-110_4 are working. To confirm each of the LED strings 110_1-110_4 has enough cross voltage while working, the minimum voltage selector 130 selects a minimum voltage among the voltages VDET1-VDET4 of the nodes N1-N4, and the voltage booster 140 determines a supply voltage Vo of the LED strings 110_1-110_4 according to the minimum voltage. In other words, the backlight module control system 100 determines the supply voltage Vo of the LED strings 110_1-110_4 by referring to the LED string having a largest cross voltage so as to make each of the LED strings 110_1-110_4 have enough cross voltage.
However, when one the LED strings 110_1-110_4 is burned out (i.e., being an open circuit), the corresponding voltage VDET will always be zero. Then, because the minimum voltage VMIN outputted from the minimum voltage selector 130 is always zero, the voltage booster 140 will continuously boost the supply voltage Vo until an over voltage protection mechanism is triggered to close the operations of the backlight module control system 100, causing the failure of the control mechanism.